1. Field of the Invention
The present invention relates to a lighting device having a surface light source device and a light guide plate, and a display device having the lighting device. More particularly, the invention relates to a lighting device using a number of point light sources such as LED (light emitting diode), and a display device having the lighting device as a surface light source device.
2. Description of Related Art
There has been proposed a display device configured to display an image by illuminating a spatial light modulating element, such as LCD (liquid crystal display), using a lighting device.
FIG. 1 is a perspective view showing the constitution of a display device using a conventional transmission TFT-LCD (thin film transistor-liquid crystal display)
FIG. 2 is a sectional view of the display device of FIG. 1.
In the display device using a non-illuminant spatial light modulating element represented by LCD, as shown in FIGS. 1 and 2, a lighting device (backlight unit) 102 is arranged on the backside of the spatial light modulating element 101. White illumination light emitted from the lighting device 102 enters picture electrodes (transparent electrodes) for red 103R, picture electrodes for green 103G and picture electrodes for blue 103B. The illumination light via the picture electrodes for respective colors is further transmitted through color filters for respective colors 104R, 104G and 104B to be red light, green light and blue light, respectively. Then, these color lights are combined to a color image for display.
As a lighting device for such a display device, for example, there is known a so-called “direct type” lighting device having a light source and a diffuser plate of which backside is opposed to the light source. In this lighting device, illumination light emitted from the light source enters the diffuser plate through its backside. Then, the illumination light emitted from the front side of the diffuser plate enters the spatial light modulating element.
As another lighting device for the display device, for example, there is also known a so-called “edge light type” lighting device having a light source and a light guide plate of which lateral side is opposed to the light source. In this lighting device, illumination light emitted from the light source enters the light guide plate through its lateral side. Then, the illumination light from the front side of the light guide plate enters the spatial light modulating element. The illumination light transmitted to the backside of the light guide plate is reflected by a reflection sheet (reflector) and returned into the light guide plate again.
Also, the lighting device may be provided, on the front side (i.e. emitting surface) of the diffuser plate or the light guide plate, with a prism lens sheet for condensing the illumination light emitted from the front side within a view angle to thereby improve brightness or a second diffuser plate (or diffuser sheet) for equalizing brightness.
As a light source in such a lighting device, there is employed a light emitting element, such as cold cathode lamp etc. forming a linear light source and LED forming a point light source. In case of light emitting elements as the light source, a plurality of light emitting elements are arranged apart from each other so as to oppose to an incident surface (backside or lateral side) of the diffuser plate or the light guide plate, as disclosed in Japanese Utility Model Registration No. 2578529 (Citation 1) and Japanese Patent Laid-Open Publication No. 10-82915 (Citation 2).
In the lighting devices of Citation 1 and Citation 2, in order to facilitate the positioning of the light emitting elements with respect to the diffuser plate (or the light guide plate), it is provided with recesses for fitting the light emitting elements. With the engagement between the recesses and the light emitting elements, they are arranged in position against the diffuser plate (or the light guide plate).